Over the years, numerous electrically powered medical devices have been implanted for applications such as control of the heart (cardiac pacemakers, defibrillators), neurostimulators, etc. Power to operate these devices is typically either a primary (non-rechargeable) battery, or a secondary (rechargeable) battery which receives its recharging power via an external coil which communicates inductively with an implanted coil.
From a user convenience perspective, it is desirable that the rechargeable battery be able to provide power to operate the implantable device for a significant period of time (1 day or more), so that recharging of the battery is only required infrequently. Additionally, it is desirable that when the recharging procedure is initiated, the battery is able to be fully recharged in as short a period of time as possible (ideally less than 1 hour). Charging is usually done by inductively coupling power from an external coil antenna through the skin to an implanted coil antenna. In order to achieve a short recharging time, high power must be applied to the external coil antenna. Such high power can lead to excessive heat being generated in the implanted coil, the implanted rechargeable battery, or the implanted electronics which control the recharging process, individually, or collectively.
The heat delivered into the components described above can result in a general rise in the temperature of the implanted device, and the temperature of the surface of the device presented to the adjacent tissue becomes a matter of concern, in order to avoid damage to the tissue.
Implanted devices must meet a requirement which limits the temperature rise of the surface of an implanted device to no more than 2° C. above the temperature of the surrounding tissue, as described in The Cenelec European Standard, EN 45502-1 (August 1997), page 18, paragraph 17.1, the disclosure of which is hereby incorporated by reference herein.
Conventional methods to regulate temperature have not proven satisfactory in meeting the conflicting requirements of the patients' convenience needs and the requirements to not damage tissue through overheating. There is, therefore, a need for an improved method and apparatus for ensuring that the surface temperature of an implanted rechargeable device does not exceed generally accepted limits.